The present invention relates to an optical fiber temperature sensor for measuring the temperature by detecting the Raman scattered light generated inside the optical fiber.
Fiber-optic technologies provide such optical sensors as used for measuring temperature, deflection, pressure and the others, in recent years, and specifically, optical fiber temperature sensors are used for sensing the temperature by using Raman scattered light generated at the optical fiber.
FIG. 4 is a schematic diagram of the circuit for the conventional optical fiber temperature sensor.
As shown in FIG. 4, the optical fiber temperature sensor 70 has the sensing part arranged at the temperature measurement point and composed of the optical fiber 71 (long-distance optical fiber for measurement) and the sensor body 72.
The sensor body 72 has the light source 73 for applying the incoming optical signal into the optical fiber 71 (for example, semiconductor laser diode (LD)), and a couple of photoelectric detectors 74 and 75 (for example, avalanche photodiode). The light source 73 and the photoelectric detectors 74 and 75 are connected to the optical fiber 71 used for sensing thorough the wavelength filter module 76. The wavelength filter module 76 has a couple of edge filters, each having its own wavelength range different from each other and branches the Raman scattered light from the back scattered light generated at the optical fiber 71.
The light source 73, being connected to LD driver 77, forms the LD module 78. A couple of photoelectric detectors 74 and 75 are connected to the received signal amplifiers 79 and 79, respectively. The received signal amplifiers 79 and 79 are connected to A/D (Analog-to-Digital) converters 80 and 80, respectively. A/D converter 80 and the LD module 78 are connected to the signal processing control circuit 81. The signal processing control circuit 81 has the signal control circuit 82 (for example, MCU (Micro Control Unit)) and the signal processing circuit 83 (for example, FPGA (Field Programmable Gate Array)), and the signal processing circuit 83 is connected to the external arithmetic processing means 84 (for example, personal computer).
The optical fiber 71 forming the sensing part is typically a multi-mode fiber or a single-mode fiber, which is generally used for communication and has a core doped with Ge and the other materials.
When injecting the pulsed optical signal generated at the LD module 78 into the optical fiber 71, the weak Raman scattered light is generated at individual places in the optical fiber 71. As shown in FIG. 5, the frequency spectrum of the Raman scattered light maps the frequencies located symmetrically below and above the center wavelength λ0 of the incoming light. The back-scattered light having the spectrum at the center wavelength λ0 of the incoming light is the Rayleigh scattered light R. The Raman scattered light with the spectrum above the center wave length is designated Stokes light St, and the Raman scattered light with the spectrum below the center wave length is designated anti-Stokes light As.
The intensity ratio between the Stokes light St and the anti-Stokes light As, both generated at the optical fiber 71, depends on the temperature of the optical fiber 71. Therefore, the temperature of the temperature-measured object affects the temperature of the optical fiber 71, and thus, contributes to the detected intensity ratio between the Stokes light St and the anti-Stokes light As. The temperature of the temperature-measured object can be measured by obtaining this intensity ratio.
In the optical fiber temperature sensor 70, the backscattered Stokes light St and the backscattered anti-Stokes light As are separated by the wavelength filter module 76, and received separately by the photoelectric detectors 74 and 75, respectively. The received light is converted into the electric signal, and then the electric signal is amplified by the received signal amplifier 79, and the amplified electric signal is converted into the digital signal by A/D converter 80, and the digital signal is supplied to the signal processing control circuit 81. The electric signal is processed by the signal processing control circuit 81 and the external arithmetic processing means 84 in order to calculate the temperature, and finally the measured temperature is displayed.